Follicular helper T (Tfh) cells are critical regulators of the humoral immune response. Tfh cells are defined by their localization within the B cell follicle or germinal center (GC), by expression of canonical surface markers and transcription factors, and by their function, enabling GC B-cell proliferation and differentiation. These phenotypes discriminate Tfh cells from the other CD4 T helper cells, such as the Th1, Th2, or Th17 populations. It is now accepted that Tfh cells are distinct from the other CD4 T-cell effector subsets. We hypothesize that even after T cells have attained the usual characteristics of Tfh cells by follicular and GC localization, surface markers and transcription factor expression, and ability to provide B cell help, they continue to mature, and that this maturation results in discree populations within the Tfh-cell pool, with each population having a unique role in directing the GC response. This hypothesis, if confirmed, would dictate a new model in which Tfh-cell development continues even after the cells have entered the GC, with this ongoing development shaping the GC response. If so, it also would have significant implications for the mechanisms leading to generation of pathogenic antibodies in autoimmune and allergic illnesses. While the factors controlling the initial development of Tfh cells are beginning to be elucidated by others and us, less is known about the fate of Tfh cells after they enter the B-cell follicle. At this poit the cells have acquired their canonical features, yet whether they retain potential to develop further is a question that remains unanswered. Our objective in this proposal is to offer a novel paradigm for continued Tfh-cell development within the GC, development that is critical for shaping the GC B response and humoral immunity. Our suggestion for this paradigmatic shift is based upon our preliminary findings that following type 2 immune challenge Tfh cells initially express IL-21 but not IL-4, and as they mature within the GC they gain IL-4 expression and lose that of IL-21, along with a change in GC location and contact-dependent interaction with B cells mediated by CD40 ligand. We outline herein experiments designed to test our hypothesis, investigating the nature of the individual populations that compose the Tfh-cell pool, and their function in GC B-cell maturation. We will, in two specific aims, identify the mechanisms regulating germinal center Tfh-cell maturation, and investigate the impact of this GC Tfh-cell transition on GC B-cell development. If successful, this examination of Tfh-cell differentiation within the GC, a phenomenon not well studied but critical for GC function, will provide a new framework for further understanding the pathogenesis of antibody formation in autoimmune and allergic diseases.